1. Field of the Invention
The invention relates to a method of operating a gas turbine power plant and in particular to a method for the operation of the gas turbine power plant during plant shut down by regulating stator vanes of a multistage axial air compressor.
2. Brief Description of the Related Art
It is generally known that during the shut down of a gas turbine, compressor instabilities can occur. In particular, stalls can occur in the compressor once lower speeds are reached. Such stalls can give rise to vibrations of the airfoils and shaft, which potentially limit the operational lifetime or lead to compressor damage.
U.S. Pat. No. 5,042,245, for example, discloses compressor stall conditions as well as measures to prevent or limit such conditions by variation of the position of stator vanes. Maintaining that a setting of vane positions as a function of rotor speed leads to unsatisfactory operation, it presents in particular a method of positioning the stator vanes together with a regulation of compressor bypass valves. The method includes a feedback loop operating according to compressor pressure ratios, which are compared to a reference compressor ratio.
This method described above is particularly directed to the application to aircraft engines in connection with a closed or open loop control.
A method of operating gas turbines at partial load, as disclosed in U.S. Pat. No. 5,634,327, includes a reduction of fuel flow to two combustion chambers and an adjustment of compressor guide blade angles at the onset of load reduction until the load drops below 50% of the rated load. At loads below 50% of rated load the fuel flow is regulated according to mass flow and inlet temperatures to the turbines.
Low speed compressor instabilities during shut down of gas turbines can also include rotational stall, which occurs when one or more neighboring compressor blades stall before other neighboring blades. This condition can lead to a stalling of blades counter to the direction of rotation of the shaft and unstalling of blades in the direction of shaft rotation. This results in a stall cell rotating at approximately 50% of the shaft speed and can lead to high shaft vibrations peaks when the shaft imbalance caused by the stall conditions coincides with a critical eigenfrequency of the shaft.